[TUHS] /dev/drum

[bakul@bitblocks.com (Bakul Shah)]

On Mon, 23 Apr 2018 22:32:26 -0600 Grant Taylor via TUHS <tuhs at minnie.tuhs.org> wrote:
> 
> I had always assumed that the outer edge (what I thought was the end of
> the disk) was faster than the inner edge (what I thought was the
> beginning of the disk) because of geometry.  However, as Ronald stated,
> hard drives were constant angular density.  Thus negating what I
> originally thought about speed.

Constant angular velocity means faster "linear" velocity for
tracks further away from the center.  Since 1990 or so disk
tracks are divided up in 16 or so "zones", where outer zones
have more blocks per track.  This translates to higher
throughput.

A modern Seagate Exos SAS disk may have a range of 279MB/s
(outermost) to 136MB/s (innermost) or 300MB/s to 210MB/s for
faster disks (15Krpm).  Disk vendors don't seem to break this
range out for consumer drives. But you can measure it using
tools like diskinfo on FreeBSD. For example:

# diskinfo -t /dev/ada4 # this is an 5 year old 1TB WD "Black" disk.
/dev/ada4
	...
        Not_Zoned       # Zone Mode <<== this seems wrong.
        ...
Transfer rates:
        outside:       102400 kbytes in   0.972176 sec =   105331 kbytes/sec
        middle:        102400 kbytes in   1.088977 sec =    94033 kbytes/sec
        inside:        102400 kbytes in   1.804460 sec =    56748 kbytes/sec